Displacement sensors, such as microphones, accelerometers, and pressure sensors, are well-known. Displacement sensors based on capacitive, impedance, and optical measurements have been developed. Optical displacement sensors are particularly attractive as they overcome many of the limitations of capacitive and impedance measurement techniques, such as low sensitivity, the need for high voltage biasing, poor electrical isolation, or response nonlinearities.
Many optical displacement sensors known in the prior art operate by detecting light reflected by an optical element that changes its reflectivity in response to an environmental stimulus, such as pressure differential, acceleration, sound, vibration, etc. A Fabry-Perot interferometer has often been used as such an optical element.
A Fabry-Perot interferometer is an optical element that comprises an optical cavity that is optically resonant for one or more wavelengths of light. A Fabry-Perot interferometer is an optical beam splitter that can receive input light and distribute it between a first output, which is reflected from the interferometer, and a second output, which is transmitted through the interferometer. The distribution of the light into these two outputs is a function of the wavelength of the input light and a variable spacing between two parallel partially-reflective surfaces that define the optically resonant cavity.
In order to form a Fabry-Perot interferometer that is sensitive to an environmental stimulus, one surface of the Fabry-Perot interferometer is a surface of a movable membrane that moves in response to the stimulus. When the movable membrane moves in response to incident sound, for example, the distribution of light between the two outputs is changed. As a result, the intensity of each of the outputs is changed. As a result, detection of one or both of these outputs by a photodetector results in an electrical signal that is a function of the acoustic energy of the incident sound.
The input light is typically generated by a coherent light source, such as a laser or LED. Such light sources are known to exhibit wavelength fluctuations due to, for example, temperature variations, drive current fluctuations, or aging. Because the response of a conventional Fabry-Perot interferometer is a function of both membrane motion and input wavelength, it can be difficult to differentiate a desired environmentally induced response from fluctuation of the wavelength of the input light. As a result, input light wavelength fluctuation represents a source of noise for the displacement sensor and reduces its sensitivity and performance. Wavelength stabilization can be used to ensure that the wavelength of the input light to the Fabry Perot interferometer remains stable. Wavelength stabilization, however, increases the complexity of the light source and can be quite expensive to implement.
An optical beam splitter that has low sensitivity to wavelength changes but also exhibits high sensitivity to an environmental stimulus would enable an optical displacement sensor having the potential for, among other things, higher sensitivity, improved signal to noise ratio, and lower cost.